net: hns: bug fix about ping6

The current upstreaming code fails to ping other IPv6 net device, because
the enet receives the multicast packets with the src mac addr which is the
same as its mac addr. These packets need to be dropped.

Signed-off-by: Kejian Yan <yankejian@huawei.com>
Signed-off-by: Yisen Zhuang <Yisen.Zhuang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>